The present invention relates generally to a method and system for detecting an encoded radio signal containing a digital code component. More specifically, the invention relates to a method and system for detecting digitally encoded radio signals. Further specifically, the invention relates to a method and system for detecting digitally encoded radio signals transmitted between a pocket-portable transmitter and a vehicle-mounted controller in an automotive keyless entry system for operating vehicle devices, such as a door lock mechanism, a trunk lid opener and so forth, without mechanical key operation or manual input of a preset code.
Recently, a novel keyless entry system for automotive vehicles has been proposed and put on the market. This system does not require mechanical key operation or manual entry of a preset code to operate various vehicle devices, such as the vehicle door lock, the trunk lid opener and so forth. In this keyless entry system, a pocket-portable transmitter is used as a source of a preset code signal. The transmitter generates radio waves carrying the preset code and transmits the radio waves to a controller mounted on a vheicle. The controller receives and decodes the radio waves. If the decoded code matches a preset code in the controller, the controller sends a control signal to the vehicle device to be operated.
In this keyless entry system, it is preferable in view of battery power consumption to employ a passive transmitter which generates and transmits the preset code-carrying radio wave in response to some trigger. In the proposed keyless entry system, a controller in the vehicle, which utilizes the power of a vehicular battery, is thus designed to generate and transmit a demand radio signal to the transmitter to trigger the latter. This demand radio signal may be a digitally encoded radio signal generated in response to manual operation of a manual switch mounted on the outside of the vehicle body, such as on an outside door handle, outside door handle escutcheon, a trunk lid ornament and so forth.
To detect the demand radio signal from the controller, the transmitter operates in a stand-by state and must somehow distinguish the demand signal from the controller from radio noise. Generally, the presence of the demand signal is detected when the received radio amplitude exceeds a predetermined threshold amplitude. However, if the received signal amplitude is simply compared with the predetermined threshold amplitude, mis-detection can easily occur at times of relatively intense noise or insufficient strong encoded radio signals, i.e. the demand signal.